Electron beam imaging requires an even charge distribution on a sample area in order to properly represent image contrast. A flood gun is used to neutralize initial charges which are carried over upon transfer of the sample to the imaging system by flooding the sample area with energetic electrons. Such flooding also equalizes charges that build up on the sample during imaging from the inspection beam. In some applications, e.g., semiconductor wafer imaging with a scanning electron microscope (SEM), the flood gun is used only during the acquisition of swath images. Frame imaging uses the electron gun to do both flooding and inspection.
The flood gun is very similar to the electron gun used in the electron beam imaging system. The main difference is that instead of emitting a focused beam concentrated on a small area (e.g., a few square microns or less), the flood gun sprays a wide beam of electrons, over a much larger area, e.g., about 1 square centimeter. The electrons from the flood gun approach perpendicular to the sample surface, flooding the sample area. Typical flood gun hardware includes a high current filament which heats a metal tip, causing emission of electrons toward an extractor electrode. The extractor controls the number of electrons which are emitted towards the wafer and can be used to turn on or turn off (blank) the beam. A Wehnelt electrode positioned close to the wafer surface creates an electric field to control the number of electrons which stay on the wafer. During inspection, the wafer floats at a potential controlled by a power supply.
Flooding and swathing typically commonly done sequentially but may be done simultaneously. Flooding and swathing generally operate on different areas of the sample. In swathing, the sample is imaged by rastering the primary beam in one direction (e.g., the X-direction) while the sample translates in a perpendicular direction (e.g., the Y-direction). By way of example, during a wafer inspection, a stage moves the sample such that an area of the sample is flooded before it is inspected. Flooding is typically either on or off for the entire duration of a swath. Because flooding covers a significantly larger area than a swath, flooding may be turned off, e.g., at least every other swath. To prolong the life of the flood gun, the flood gun beam may be blanked, e.g., by applying a suitable voltage to the extractor, when not needed.
It is within this context that aspects of the present disclosure arise.